The present invention relates generally to oscillators used in communication transceivers, and more particularly, to direct-coupled differential oscillators.
A communication transceiver utilizes locally generated oscillator signals to frequency translate modulated message signals. Ideally, the oscillator signals are single-frequency, sinusoidal signals without any jitter or noise. Furthermore, these oscillator signals are generally controlled by phase-locked loops and are designed to span the range of communication channels.
In practice, the oscillator signal contains noise. The noise affects receiver systems by shifting nearby interfering signals to the same spectrum as the wanted signal. It also degrades transmitter systems by lowering the signal-to-noise ratio and modulation accuracy of the message signal.
In its simplest form, an oscillator consists of a resonant circuit with positive feedback. The resonator sets the frequency of the output signal and shapes the noise produced by the oscillator, as shown by:             s      θ        ⁡          (              Δ        ⁢                  xe2x80x83                ⁢        ω            )        =                    F        ⁢                  xe2x80x83                ⁢        k        ⁢                  xe2x80x83                ⁢        T                    P        s              ⁡          [              1        +                              Δ            ⁢                          xe2x80x83                        ⁢            ω                                              (                              2                ⁢                                  xe2x80x83                                ⁢                Q                ⁢                                  xe2x80x83                                ⁢                ω                            )                        2                              ]      
where Sxcex8(xcex94xcfx89) is the oscillator phase noise at an offset frequency xcex94xcfx89, F is a noise scaling factor, k is Boltzman""s constant, T is temperature, Ps is the carrier signal power, Q is the resonator""s quality factor, and xcfx89 is the frequency of the carrier signal.
Lower phase noise can be obtained by improving circuit topologies and increasing DC power, which lowers the noise-scaling factor (F) and increases the amplitude of the oscillator""s output signal. These factors are generally limited by operating voltage and device technology. Thus, it would be desirable to have a way to control the amplitude of the oscillator""s output without the above-described limitations.
The present invention includes an oscillator circuit with feedback to accurately control the circuit""s output amplitude. The oscillator circuit minimizes phase noise by enabling high amplitude levels without device saturation or breakdown. The feedback loop controls the oscillator""s output amplitude and thereby simplifies system and circuit design. Amplitude control of the oscillator""s output signal provides several advantages. It prevents transistor saturation and breakdown, enables optimization of mixer signal levels, accurately sets the output level in direct synthesis transmitters, and allows lower power consumption in adaptive receivers operating in strong signal environments.
In one embodiment of the present invention, a high-swing oscillator is provided. The oscillator comprises a resonator circuit and a pair of transistors coupled to the resonator circuit, the transistor pair having collector terminals coupled to a power supply and base terminals selectively biased to prevent saturation or breakdown.
In another embodiment included in the present invention, a feedback loop is provided for use with a differential direct-coupled oscillator circuit to control the amplitude of an oscillator output signal. The feedback loop includes a pair of clamping transistors, wherein base terminals of the clamping transistors are coupled to an adjustable voltage signal to prevent saturation of the oscillator circuit. The loop also includes a filter to monitor the current flowing through the clamping transistors. The loop also includes an amplifier to compare an output of the clamping transistors to a reference signal, and a reference generator to set an operating bias for the direct-coupled oscillator.
In another embodiment included in the present invention, a method is provided for controlling the output amplitude of a differential direct-coupled oscillator. The method comprises the steps of clamping a low peak of an output signal of the oscillator, monitoring a clamping current, converting the clamping current to a filtered voltage, comparing the filtered voltage to a reference voltage to produce a difference voltage, and setting a bias current in the oscillator using the difference voltage.